Non-native species invasions
The introduction of harmful aquatic organisms to new marine environments is believed to be one of the four greatest threats to the world's oceans. Together with overexploitation, it has been identified as the major cause of species extinction (Bellard et al., 2016). An alien or non-native species is one that has been intentionally or accidentally transported and released into an environment outside of its historic or resident geographical range or habitat. Such species are described as 'invasive' if they are ecologically and/or economically harmful. Invasive species can dramatically change the structure and function of marine ecosystems by changing biodiversity and eliminating vital components of the food chain.
Functional integrity of species communities
Even though all seas are interconnected, there are certain dispersion barriers such as salinity, temperature gradient or ocean currents that keep local communities distinct. Two identical habitats can have different species communities with different types of interactions contributing to the global richness of biodiversity. Indigenous, or native species are those living within their natural range (past or present) including the area that it can reach and occupy using its natural dispersal system. By contrast, introduced species are transported either intentionally or accidentally by human-mediated vectors into habitats outside their native range. These species are also termed as alien, exotic, invasive, foreign, non-native, immigrant, neobiota, naturalized, or non-indigenous. Some biota cannot be sufficiently proved to be neither native nor exotic and these are termed cryptogenic species (Carlton, 1996 )
Through the evolutionary fitness processes, species adjust to each other and adapt to available resources by occupying different ecological niches within communities. Virtually all resources are utilized optimally and all available niches are filled, maximising the biodiversity value. Various factors influence the functional integrity of a community. If changes are occurring gradually over a long timescale species have enough time to adapt and fill available niches. In turn more rapid shifts create niche openings and this has been identified as the main prerequisite of species invasion. Non-native species are found primarily in disturbed areas, such as harbours, bays, estuaries and semi-enclosed seas where the communities are weakened by various types of pollution (Elton, 1958; Cohen, 2004).
Vectors of introduction
Whether deliberately or accidentally, people have been transporting whole range of organisms, breaking natural distribution boundaries and interfering with community structures. The unwanted hitchhikers are usually either well-hidden or too small to be noticed – for the entire lifespan or just for its part.
For most coastal species the open ocean environment is inhospitable, preventing them from spreading into habitats similar to their own but located elsewhere. Distances separating such habitats might be too long to overcome either through their active swimming abilities or passive floating in water currents. Mechanisms by which humanity aids introduction of exotic species are called vectors of introduction and these are chiefly associated with shipment activities (hull fouling and ballast water) and marine aquaculture (Bax et al., 2003). Other vectors include ornamental species trade, the international transport and sale of live marine bait, live seafood, and live organisms for research and education.
Organisms can travel either attached to the submerged part of the vessels hull or contained within their ballast waters. Ballast water has been extensively used since 1870s and certain species are able to complete their life cycle and breed in ballast waters, meaning that they can be translocated far away from their native range. Studies on ballast tanks found more than thousand taxa ranging from phytoplankton to small fish up to 15 cm in length (Gollash et al., 2004).
In order to combat the global dispersion of species, an international initiative was launched in 2004 for a Ballast Water Management Convention, which came into effect in 2017. Under the Convention, ships are required, according to a timetable of implementation, to comply with the D1 or D2 standards . The D1 standard requires ships to carry out a ballast water exchange, and specifies the volume of water that must be replaced. This standard involves exchanging the uptaken discharge water from the last port, with new sea water; it must occur at a minimum of 200 nautical miles from shore. The D2 standard is more stringent and requires the use of an approved ballast water treatment system. The main types of ballast water treatment technologies are : Filtration systems (physical); Chemical disinfection (oxidizing and non-oxidizing biocides); Ultra-violet treatment; Deoxygenation treatment; Heat (thermal treatment); Acoustic (cavitation treatment); Electric pulse/pulse plasma systems; Magnetic Field Treatment.
There has been a growing interest in the search for fish, shellfish, and plant species whose biology was well known and which either already had achieved or could achieve success in cultivation. Aquaculture is the world's fastest expanding food production sector, with an annual growth rate of 37% recorded in 2016 and a projected production of 109 million tonnes by 2030 (FAO, 2018). Depending on the type of mariculture, the organisms can be either allowed to establish in the wilderness (intentional species introduction) or kept in enclosures from which they occasionally escape. It is estimated that 26.5% of global fish farming comprises non-native species, equating 1.74 million tonnes per year and that nearly a third of marine ecoregions of the world are to some extent at risk from the impacts of fish escapes (Atalah and Sanchez-Perez, 2020). Escapees are the result of, among other things, net failures, storms and accidents at sea.
- Ornamental species trade
Saltwater species are a rapidly growing sector of the aquarium industry. Species can be released by their owners either accidentally or deliberately when no longer wanted. This vector is responsible for one of the major invasions launched up to date, namely the release of Caulerpa taxifolia from Oceanographic Museum of Monaco.
- Indirect introduction
Occasionally, species introduction can result not from their physical re-location but by offering a way for its dispersal to areas that it wouldn’t be able to reach if the conditions were not changed. Such opportunity is offered by alteration of hydrological regimes, like canal and reservoir construction (Grigorovich et al., 2002).
- Secondary introduction
If species are introduced and become established in a new geographic setting, they might continue to spread by both natural and anthropogenic mechanisms, colonizing habitats that they wouldn’t be able to reach without the initial, human-mediated translocation. Such introduction is termed a secondary introduction.
Very rarely we can connect an invasion event with one particular vector. Instead it is more common to assess the probability of a given vector being responsible. In British waters accidental release associated with mariculture has been identified as the main vector with other important vectors being fouling, ballast water and deliberate commercial introduction (Eno et al., 1997).
From establishment to invasion
Not all translocated species become established in new environments. If the population is relatively small it will be vulnerable to stochastic threats such as demographic or genetic drift. It is hard to predict what is the minimum viable population size. A generally accepted rule is that 50 individuals are needed to prevent excessive inbreeding and a minimum population of 500 hundred is required to keep a sufficiently high level of heterozigosity (Franklin 1980). Yet, those numbers may very greatly between different taxa and even if the population is large enough to prevent loss of alleles, it still might be prone to environmental threats such as poor food or oxygen availability, impacting recruitment success or juveniles development, or catastrophic events (El Niño, tsunami, etc.)
If the founder population is large enough to overcome stochastic threats and manages to establish itself in a new environment, it will join the network of interactions within the receiving environment. Alien species have evolved in different environments and it is hard to predict what their interactions with indigenous biota will be. Not all introduced species can be termed as invasive. Some of them may well coexist with native species and share the resources. But every now and then an introduced species becomes an invader and impacts the host community, with the ultimate result being destabilization of the system and possibly extinction of native species.
Organisms have been carried around hidden in dry ballast, attached to hulls or buried inside them for millennia and it is very likely that what we now consider cosmopolitan species are simply early introductions and that species such as ship-boring isopod (Sphaeroma terebrans), Asian seasquirt (Stylea plicata), giant kelp (Macrocystis pyrifera), mussels (Mytilus galloprovincialis and Mytilus edulis) and European periwinkle (Littorina litorea) can be possible early introductions (Carlton, 1999). This in turn raises the issue of naturalisation: How much time is needed to fully incorporate a population in a community and bring the latter to equilibrium? How many interactions have to be established to call an introduced species integrated? Carlton also questioned the quality of our understanding of present marine communities. If we assumed a rather timid scenario that between 1,500 and 1,800 only three species were introduced elsewhere each year, we would have ended up with a number of thousand species that might have spread before humanity gained a general knowledge of biogeography, taxonomy and ecology. As a result, these species can be today considered as cosmopolitan. It might have a great impact on our understanding of marine ecology and community equilibrium in particular.
Introduced species might have evolved in the presence of much more aggressive competitors than the ones present in the receiving environment and they might be extremely successful in colonising new areas. The most common competition is for resources, whether it is food, solar energy or space. In California bay the native shore crab (Hemigrapsus oregonensis) declined in mean abundance by 10 times as a consequence of the European green crab (Carcinus maenas) introduction and competition for food (Nutricola sp.) (Grosholz et al., 2000).
- Diffuse competition
The impact on native organisms can be greater through diffuse competition. A population might resist competition along one axis if the resource is plentiful, but its realised niche can virtually disappear in the presence of several species competing along many axes (Giller, 1984).
- Positive indirect interactions
Even with a stable or decreasing rate of new introductions, it can be assumed that systems will become increasingly destabilized through positive indirect interactions and diffuse competition, as more and more interactions within the community become altered. Even early, non-invasive introductions may become a nuisance due to a change induced by another introduced species (Grosholz, 2005). It is possible for a new alien to transform an older exotic species into an invader.
- Predation and herbivory
Some organisms are more effective competitors for resources than others and have the potential to dominate the area, excluding other species from the access to viable resources. In such cases organisms feeding on it (predators or herbivores) act as a biocontrol agent keeping their abundance low and maintain a high level of stability and productivity. If those predators or herbivores are introduced to a new environment, local community members might be lacking the evolutionary-driven ability to resist them. On the other hand if a species limited in number in its native range by biocontrol agents is introduced to a new environment, it might thrive and outcompete indigenous species.
Very often introduced organisms are accompanied by parasitic species. Mass transfer of large numbers of animals and plants without inspection, quarantine, or other management procedures has led to the simultaneous introduction of pathogenic or parasitic agents. Fish escaped from maricultures may carry along parasitic organisms that could later establish themselves in indigenous species (Kent, 2000), especially when farmed and wild fish of the same or closely related species are in close vicinity (Arechavala-Lopez et al., 2013). Since the wild fish have not developed any defence strategy against non-indigenous parasites, they could be greatly impacted.
- Genetic impact
If transferred species are closely related to native counterparts, they may hybridise affecting the original gene pool. This can especially happen with escapees from fish farms (Bolstad et al., 2017). Introduced or hybridised individuals may prove to be more successful in mating than the indigenous organisms and the original genetic diversity might be lost. Most farmed fish have low genetic diversity as a result of selective breeding for favourable production traits over several generations. Thus, genetic introgression of farmed fish can result in alterations in the genetic composition, long-term loss of fitness, adaptability and reduced survival of wild fish populations (Miralles et al., 2016).
- Multiple negative interactions
Two species may interact with one another in a several ways. A species confronted by one-way interactions can compromise and trade-off one component of fitness for the sake of the survival. If it is forced to compromise too many components it may be pushed into an extinction vortex (Russel et al., 2004).
Consequences of human-induced altering of species composition are sometimes detrimental to local communities and their magnitude may range from limitation or exclusion of single species or to destabilization of the whole system and species introduction has been identified as one of the main causes of species extinction. Even with a stable or decreasing rate of new introductions, it can be assumed that systems will become increasingly destabilized through direct and indirect interactions and diffuse competition as more and more interactions within the community become altered. In areas that have already been heavily invaded, reducing the numbers of new introductions may not be a sufficient strategy. In addition to preventing new introductions, it may be necessary to mitigate the impacts of exotic species that have already become established. Given the large number of alien species already present, there is a high potential for positive interactions to produce many future management problems.
At least 50 non-native species have entered the Black Sea in the last century and some have been invasive. For instance, the comb jelly-fish Mnemiopsis leidyi was the primary cause of collapse of the fisheries in the area in the early 1990s. Over 100 non-native species have been recorded in the NE Atlantic, mainly in the North Sea, the Celtic Sea, the Bay of Biscay and along the Iberian coast. Impacts of invasive species vary in different regions and sometimes are rather localized. Over the past twenty years, the number of alien species transported into the Baltic has increased and poses a significant threat to the region given its naturally low species diversity. In North America, some 300 non-indigenous species of invertebrates and algae have been established in marine and estuarine waters (Ruiz et al., 2000). Rates of invasion among most microscopic organisms such as bacteria are still unreported and it appears that their potential of dispersal by human-mediated vector is very significant.
- Threats to the coastal zone
- Species extinction
- Ecological thresholds and regime shifts
- Resilience and resistance
- Marine invasive species
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